Fix check of the size of the alpha identifier

This change fixes an issue that FDN number is unexpectedly overwritten
by wrongly encoded FDN name. The issue can happen if user enter FDN name
which contains multibyte character.

That is because AdnRecord does not check the length of the alpha
identifier correctly. mAlphaTag.length() does not return the size of the
alpha identifier but returns the number of characters.

Change-Id: I109a01eea6519a6a09a8bf7a143eaafd8c04025e