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
Change-Id: Ic90e1bd6615f5954577ed29c3daf7b44596fb137