Validate IME metadata before parsing

Malicious IMEs can load an extremely large metaData for inputmethod xml
which can lead to IMMS running out of memory while contructing
InputMethodInfo.
this change limits the size of metadata xml to 200KBs and would throw
XmlPullParserException for xmls larger than that.

Bug: 416259832
Test: Manually using steps in the bug
Flag: EXEMPT bug_fix
(cherry picked from commit 5e31d9c0)
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:b417c7c9102e5510da35dbfc771eab3fe1dc670d
Merged-In: Ic90e1bd6615f5954577ed29c3daf7b44596fb137
Change-Id: Ic90e1bd6615f5954577ed29c3daf7b44596fb137