httplive: fix use-after-free

Implement a mutex to ensure secure multi-threaded
access to the KeyedVector in MetaDataBase.
Concurrent access by different threads can lead
to accessing the wrong memory location due to
potential changes in the vector

Bug: 278166920
Test: HTTP Live Streaming test
(cherry picked from https://partner-android-review.googlesource.com/q/commit:a2dfb31957a9d5358d0219a0eda7dcb5b0fff5fe)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:90fb4ca425444429ada6ce0de1c13d35829bc196)
Merged-In: Id35ba181185bc93d9f268309a1514c5a18166e12
Change-Id: Id35ba181185bc93d9f268309a1514c5a18166e12