Parse "private" methods/fields in text ApiFile parser

CTS test of private APIs requires to parse their list generated
by API writers in CL I6e5eda65237fd80275842508d1580cbf961fa17d.
This patch adds "private" to the list of recognized keywords by
the ApiFile parser.

Test: m cts
Bug: 64382372
Change-Id: Iab10298f8ab795ab471ee58afa8f9e9b93a911f7
1 file changed