Sync with debian upstream

See merge request Librem5/debs/olm!3

add patch cherry-picked upstream to only export olm functions to avoid colliding with other libraries; closes: bug#990006, thanks to Evangelos Ribeiro Tzaras

Update to upstream version '3.2.3~dfsg'
with Debian dir d39b93f87db0b6f0aa62df049c857ab9426881fb

olm::decode_base64 now returns the length of the raw decoded data on
success. When given input with an invalid base64 length, it fails early
(before decoding any input) and returns -1.

This also makes the C function _olm_decode_base64 an actual binding of
olm::decode_base64 instead of a wrapper with slightly different
behaviour.

As a precaution.

_olm_decode_group_message should initialize all fields of the results
struct before returning. This is because its caller
_decrypt_max_plaintext_length relies on it having initialized these
fields.

Luckily, this only allows one to subvert the version check in
_decrypt_max_plaintext_length, but not the following check that the
ciphertext field is non-null because that field *is* initialized.

Consider the case when the input is size 0. In this case, `count` and
`buffer_pos` will be 0 as well. The `realloc` call in the `count == 0`
branch will then effectively become a free.

However, `realloc` can sometimes return `NULL` when a 0 is passed for
the size. The current code assumes that this only happens on a memory
allocation error and breaks out of the loop. This then becomes a double
free because the buffer is freed a second time, causing an abort.

The intent of the `realloc` is probably to downsize the buffer to fit
the data exactly in order to make incorrect memory access more obvious.
This commit skips this downsizing if the size of the input data is 0.

EXTRA_EXPORTED_RUNTIME_METHODS is deprecated. Replace with
EXPORTED_RUNTIME_METHODS.

Fix URL to the NCC Group audit.

See merge request matrix-org/olm!22

The original URL is now redirecting to a generic listing page and there
are no links to the actual Olm audit paper there.

Signed-off-by: Lukas Lihotzki <lukas@lihotzki.de>

Update gradle wrapper and build tools

See merge request matrix-org/olm!20

Using command `./gradlew connectedAndroidTest` on an API 21 emulator

BuildConfig.VERSION_NAME is not available anymore when building library
Also replace JCenter by MavenCentral

Xcode: Add support of Swift Package Manager

See merge request matrix-org/olm!19

Made by Johennes at https://github.com/matrix-org/olm/issues/51#issuecomment-809128833